The projection of bacteriophage proline and serine transfer RNA molecules represents the best understood example of a biosynthetic pathway leading from DNA to tRNA. The seven terminal steps involved in the synthesis of these molecules has been defined by mutant methodology and confirmed by a construction of the reactions in vitro from purified precursor RNA intermediates and enzymes. This work has given us some appreciation of the interactions between precursor RNA's and the enzymes that recognize and handle these molecules. The next phase of this work will be directed at obtaining a more comprehensive understanding of the molecular features underlying the specificity of these interactions. This effort is likely to be assisted by the availability of mutants altered in specific interactions, together with a knowledge of the 3-dimensional structure of the relevant parts of the precursor RNA molecules. In addition, the biosynthetic pathway will be extended back toward the DNA to include a description of the initial steps following transcription of the DNA sequences.